1. Field of the Invention
The present invention relates generally to systems and methods for assaying biological samples. Particularly, the present invention relates to a system and method for separating plasma from whole blood and transferring measured amounts of the plasma to a test substrate.
Many diagnostic markers and other analytes are best determined from patient blood samples. The ability to measure such markers and analytes directly in blood, however, is problematic because of the presence of blood cells, particularly red blood cells, in whole blood. Thus, a majority of analytical systems and methods intended for measuring blood markers and analytes rely on detection in a plasma or serum sample, not in a whole blood sample.
A wide variety of blood separation systems and methods have been developed over the years particularly for use in performing plasma and serum assays. Such systems may be generally categorized as centrifugal, chemical, and filtration. Centrifugal systems rely on centrifugation to remove cellular components from the blood. Chemical systems rely on antibodies, lectins, or the like, for binding and removing cellular components. Finally, filtration relies on chromatographic and/or size-exclusion filtration elements for separating the cellular blood components from the resulting plasma fraction.
The present invention is concerned in particular with filtration systems and methods for producing plasma from whole blood prior to performing an assay on the plasma. Heretofore, filtration elements have often been constructed integrally in assay devices, where blood is applied to the filter element and plasma is separated and flows to a reaction zone, usually without precise measurement of the plasma and without the ability to separate the excess blood and cellular blood components from the assay device. Such prior assay devices were often limited in their ability to provide diluents, eluents, and other liquid reagents in the measurement system. Furthermore, such prior devices were frequently limited to a single reaction zone for determining a single marker or analyte.
For these reasons, it would be desirable to provide improved systems and methods for separating plasma from whole blood for use in assays and assay devices. It would be particularly desirable if such systems and methods could provide for accurate and precise (receptacle) measurement of the plasma being separated as well as for direct transfer of the measured plasma volume to a test substrate. It would be further desirable if systems and methods allowed for complete separation of the excess blood and cellular components from the separation device, as well as for subsequent elution of the separated plasma to one or more associated reaction zones. In particular, it would be desirable if the systems and methods of the present invention provided for assays employing a plurality of reaction zones capable of measuring a multiplicity of analytes from a single blood sample, particularly from a low volume blood sample of 500 .mu.l or below.
2. Description of the Background Art
Filter elements for separating plasma from whole blood in combination with absorptive pads are shown in a number of patents, including U.S. Pat. Nos. 5,262,067; 5,132,086; 5,110,724; 4,987,085; 4,933,092; 4,816,224; 4,753,776; 4,477,575; and 3,663,374. In particular, U.S. Pat. No. 5,132,086 shows a device where sample is absorbed into a filtering pad, optionally by dipping the pad into the sample to obtain a semiquantitative volume of sample, and sample then allowed to flow through a reaction zone on an adjacent absorptive strip. U.S. Pat. No. 5,110,724 shows a device for separating plasma from whole blood and distributing the plasma over an absorptive distribution strip. The plasma is then transferred to a plurality of separate reaction pads by simultaneously contacting the pads with the distribution strip. U.S. Pat. No. 4,987,085 shows a device having a cell-filtering structure formed over an absorptive pad. Blood is applied to the filtering structure, and the filtering structure removed after plasma has been absorbed in the pad. U.S. Pat. Nos. 4,816,224 and 4,477,575 describe a device having a cell-filtering structure removably formed over an absorptive reaction layer. Other patents showing assay devices including separation membranes and absorptive pads include U.S. Pat. Nos. 4,678,757; 4,952,516; 4,999,163; 5,000,922; 5,135,719; 5,186,843; 5,266,219; and 5,308,483.